Various MEMS devices are becoming increasingly popular. MEMS transducers, and especially MEMS capacitive microphones, are increasingly being used in portable electronic devices such as mobile telephones and portable computing devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, the read out is usually accomplished by measuring the capacitance between a pair of electrodes which will vary as the distance between the electrodes changes in response to sound waves incident on the membrane surface.
FIGS. 1a and 1b show a schematic diagram and a perspective view, respectively, of a known capacitive MEMS microphone device 100. The capacitive microphone device 100 comprises a membrane layer 101 which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode 102 is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 103 is mechanically coupled to a generally rigid structural layer or back-plate 104, which together form a second capacitive plate of the capacitive microphone device. In the example shown in FIG. 1a the second electrode 103 is embedded within the back-plate structure 104.
The capacitive microphone is formed on a substrate 105, for example a silicon wafer which may have upper and lower oxide layers 106, 107 formed thereon. A cavity 108 in the substrate and in any overlying layers (hereinafter referred to as a substrate cavity) is provided below the membrane, and may be formed using a “back-etch” through the substrate 105. The substrate cavity 108 connects to a first cavity 109 located directly below the membrane. These cavities 108 and 109 may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes 102 and 103 is a second cavity 110.
The first cavity 109 may be formed using a first sacrificial layer during the fabrication process, i.e. using a material to define the first cavity which can subsequently be removed, and depositing the membrane layer 101 over the first sacrificial material. Formation of the first cavity 109 using a sacrificial layer means that the etching of the substrate cavity 108 does not play any part in defining the diameter of the membrane. Instead, the diameter of the membrane is defined by the diameter of the first cavity 109 (which in turn is defined by the diameter of the first sacrificial layer) in combination with the diameter of the second cavity 110 (which in turn may be defined by the diameter of a second sacrificial layer). The diameter of the first cavity 109 formed using the first sacrificial layer can be controlled more accurately than the diameter of a back-etch process performed using a wet-etch or a dry-etch. Etching the substrate cavity 108 will therefore define an opening in the surface of the substrate underlying the membrane 101.
A plurality of holes, hereinafter referred to as bleed holes 111, connect the first cavity 109 and the second cavity 110.
As mentioned the membrane may be formed by depositing at least one membrane layer 101 over a first sacrificial material. In this way the material of the membrane layer(s) may extend into the supporting structure, i.e. the side walls, supporting the membrane. The membrane and back-plate layer may be formed from substantially the same material as one another, for instance both the membrane and back-plate may be formed by depositing silicon nitride layers. The membrane layer may be dimensioned to have the required flexibility whereas the back-plate may be deposited to be a thicker and therefore more rigid structure. Additionally various other material layers could be used in forming the back-plate 104 to control the properties thereof. The use of a silicon nitride material system is advantageous in many ways, although other materials may be used, for instance MEMS transducers using polysilicon membranes are known.
In some applications, the microphone may be arranged in use such that incident sound is received via the back-plate. In such instances a further plurality of holes, hereinafter referred to as acoustic holes 112, are arranged in the back-plate 104 so as to allow free movement of air molecules, such that the sound waves can enter the second cavity 110. The first and second cavities 109 and 110 in association with the substrate cavity 108 allow the membrane 101 to move in response to the sound waves entering via the acoustic holes 112 in the back-plate 104. In such instances the substrate cavity 108 is conventionally termed a “back volume”, and it may be substantially sealed.
In other applications, the microphone may be arranged so that sound may be received via the substrate cavity 108 in use. In such applications the back-plate 104 is typically still provided with a plurality of holes to allow air to freely move between the second cavity and a further volume above the back-plate.
It should also be noted that whilst FIG. 1 shows the back-plate 104 being supported on the opposite side of the membrane to the substrate 105, arrangements are known where the back-plate 104 is formed closest to the substrate with the membrane layer 101 supported above it.
In use, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane is deformed slightly from its equilibrium or quiescent position. The distance between the membrane electrode 102 and the backplate electrode 103 is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown). The bleed holes allow the pressure in the first and second cavities to equalise over a relatively long timescale (in acoustic frequency terms) which reduces the effect of low frequency pressure variations, e.g. arising from temperature variations and the like, but without impacting on sensitivity at the desired acoustic frequencies.
The membrane layer and thus the flexible membrane of a MEMS transducer generally comprises a thin layer of a dielectric material—such as a layer of crystalline or polycrystalline material. The membrane layer may, in practice, be formed by several layers of material which are deposited in successive steps. Thus, the flexible membrane 101 may, for example, be formed from silicon nitride Si3N4 or polysilicon. Crystalline and polycrystalline materials have high strength and low plastic deformation, both of which are highly desirable in the construction of a membrane. The membrane electrode 102 of a MEMS transducer is typically a thin layer of metal, e.g. aluminium, which is typically located in the centre of the flexible membrane 101, i.e. that part of the membrane which displaces the most. It will be appreciated by those skilled in the art that the membrane electrode may be formed by depositing a metal alloy such as aluminium-silicon for example. The membrane electrode may typically cover, for example, around 40% of area of the membrane, usually in the central region of the membrane.
Thus, known transducer membrane structures are composed of two layers of different material—typically a dielectric layer (e.g. SiN) and a conductive layer (e.g. AlSi).
Typically the flexible membrane layer 101 and membrane electrode 102 may be fabricated so as to be substantially planar in the quiescent position, i.e. with no pressure differential across the membrane, as illustrated in FIG. 1a. The flexible membrane layer may be formed so as to be substantially parallel to the back-plate layer in this quiescent position, so that the membrane electrode 102 is parallel to the back-plate electrode 103. However, over time, the membrane structure may become deformed—e.g. as a consequence of relatively high or repeated displacement—so that it will not return to exactly the same starting position.
A number of problems are associated with the previously considered transducer designs. In particular both the membrane and the membrane electrode will suffer intrinsic mechanical stress after manufacture, for instance due to being deposited at relatively high temperatures of a few hundred degrees Celsius and desiring on return to room temperature to contract by different amounts due to greatly different thermal coefficients of expansion yet being intimately mechanically coupled together. Not being able to immediately dissipate the stored energy due to the stress, i.e. not able to fully release the stress by independent mechanical contraction, the composite structure of electrode and membrane will tend to deform, similar to the well-known operation of bi-metallic strip thermostat sensors. Over a long time, especially when subject to repeated mechanical exercising as typical of a microphone membrane in use, the metal electrode layer in particular may be subject to creep or plastic deformation as it anneals to reduce its stored stress energy—being unable to release it in any other way. Thus, the equilibrium or quiescent position of the membrane structure comprising the flexible membrane and the membrane electrode is sensitive to manufacturing conditions from day one and can also change over time.
FIG. 2 illustrates the permanent deformation which can occur to the quiescent position of the membrane 101/102 over time. It can be seen that the quiescent position of the membrane, and thus the spacing between the back-plate electrode 103 and the membrane electrode 102, therefore changes from its position immediately after manufacture—shown by the dashed line—to the deformed quiescent position. This can lead to a DC offset in the measurement signal from such a transducer, as the capacitance at the quiescent position is not the same. More importantly, for a.c. audio signals, the change in capacitance leads to a variation in the signal charge for a given acoustic stimulus, i.e. the acousto-electrical sensitivity of the microphone.
In addition, the elasticity of the composite electrode-membrane structure 101/102 is sensitive to the mechanical stress of the electrode and membrane layers. Any variation in manufacturing conditions and the subsequent stress release via metal creep or suchlike will affect the values of the stress of these layers. The deformation due to the stress mismatch will also directly affect the values of quiescent stress.
Thus, it can be appreciated that the membrane structure and associated transducer may suffer an increased manufacturing variation in initial sensitivity and furthermore experience a change—or drift—in sensitivity over time meaning that the transducer performance cannot be kept constant.
Furthermore, the metal of the membrane electrode may undergo some plastic deformation as a consequence of relatively high or repeated displacement from the quiescent/equilibrium position. Thus, the metal of the membrane electrode may be deformed so it will not return to its original position. Since the flexible membrane 101 and the membrane electrode 102 are mechanically coupled to one another this can also lead to an overall change in the quiescent position of the flexible membrane 101 and/or a change in the stress properties and thus the elasticity of the overall membrane structure.